Define the operation $\S$ as follows: $a\,\S\, b=3a+5b$. What is the value of $7\,\S\,2$?
Explanation: The problems asks us to substitute $7$ for $a$ and $2$ for $b$ in the expression $3a+5b$.  We find that $7\S 2=3(7)+5(2)=21+10=\boxed{31}$.